The neurons of the primary visual cortex are highly selective for specific features of the visual image such as orientation, size, depth, and direction of motion. How this integration is accomplished by the intricate connections of the cortical neurocircuitry is the subject of this proposal. The mechanisms of integration will be studied by recording intracellularly from cortical neurons in the intact animal and analyzing the synaptic potentials evoked by visual stimulation. These potentials give a direct indication of what type of visual information a neuron receives, from which presynaptic neurons it receives that information, and by what mechanisms the neuron processes the information. Four series of experiments are proposed for the next 5 years of the project. 1) The precise spatial geometry of the receptive fields of simple cells will be mapped using rapidly flashing small stimuli. From these maps, the orientation selectivity of the neurons will be predicted with a linear model, and the predictions compared to the measured orientation selectivity of the neurons. The results will help to evaluate the merits of two competing models of cortical function. 2) We will measure changes in orientation selectivity that occur during the early phases of the response to flashing stimuli. A recently introduced method will be used to make precise measurements at times when the responses are just emerging from the background. These measurements will be compared to the predictions of orientation selectivity obtained in first experiment. 3) We will explore the cellular mechanisms underlying the phenomenon of contrast adaptation, in which prolonged exposure to a pattern renders the nervous system less and less sensitive to that pattern over time. Two possible mechanisms will be explored: changes in the electrical properties intrinsic to the affected neuron, and changes in the efficacy of the synaptic input to the neuron. 4) Although they are the primary determinate of cortical function, the properties of individual synapses in the cortex have never been studied in the intact animal. We will do so by recording intracellularly from nearby pairs of cortical neurons, searching for synaptic potentials triggered in one neuron by spikes in the other.